Aqueous coating materials are used in particular to produce two-coat automotive topcoat systems of the basecoat-clearcoat type. In such systems, coating materials according to the invention are particularly the basecoat materials or basecoats. The quality of a paint system obtainable by a process for producing two-coat paint systems depends critically on the aqueous basecoat employed.
EP-A-353 797 describes aqueous coating materials which can be used as basecoats. The binder present in these known coating materials is a polymer obtainable by subjecting acrylate and/or methacrylate monomers to an emulsion polymerization, initiated by means of water-soluble initiators in the presence of an anionic polyurethane resin, which may also contain vinyl groups. If the aqueous coating materials described in EP-A-353 797 are used as basecoats in the process referred to above, the resulting two-coat paint systems are unsatisfactory in their stability to condensed moisture. This disadvantage is evident in particular with refinishes, which are cured only at temperatures of up to 80.degree. C. Furthermore, the aqueous coating materials described in EP-A-353 797 display unsatisfactory stability on storage if they include a melamine resin as additional binder component.
EP-A-297 576 describes a process for producing two-coat paint systems of the type described above in which the basecoat used comprises aqueous coating materials comprising an aqueous polymer dispersion obtainable by polymerizing ethylenically unsaturated monomers in an aqueous dispersion in the presence of a polyurethane resin which contains urea groups but no vinyl groups. If the aqueous coating materials described in EP-A-297 576 are used as basecoats in the process referred to above for producing two-coat paint systems, then the two-coat paint systems obtained are in need of improvement in terms of their resistance to condensed water. Furthermore, the aqueous coating materials described in EP-A-297 576 are frequently observed to show deficient stability on storage and defects whose root cause is incompatibility, if combinations of different binders are used.
DE-A-40 10 176 describes a process for producing a two-coat paint system using an aqueous basecoat comprising as binder a polymer obtainable by polymerizing ethylenically unsaturated monomers in an organic solvent in the presence of a polyurethane resin containing polymerizable double bonds and converting the resultant reaction product into an aqueous dispersion. If basecoats containing metal pigments are used in the process described in DE-A-40 10 176, the resulting two-coat metallic systems are in need of improvement in terms of their metal effect. Finally, the stone-chip resistance is also capable of improvement.
The reference DE P 4339870.7 discloses an aqueous coating material based on an aqueous dispersion of a polyurethane resin which is polymerized with ethylenically unsaturated monomers in the presence of a water-insoluble initiator. A lack of sufficient shear stability means that this known coating material is poorly suited to the dispersion of pigments.
The references EP 522419 and EP 522420 disclose the polymerization of monomers in the presence of polyurethane dispersions containing polymerizable double bonds. In accordance with these references, the polymerization is conducted using water-soluble initiators, resulting in unsatisfactory stability on storage of coating materials prepared using such binders. The polymerization, moreover, is carried out in the aqueous phase, leading to binders of comparatively poor suitability for the dispersion of pigments.
The technical problem on which the invention is based is to provide a novel aqueous coating material for producing two-coat paint systems of the type referred to above, with which paint systems are obtained whose technical properties are improved in comparison with the prior art and which, in particular, feature the above-described disadvantages of the prior art to a reduced extent, if at all.